I fixed the style concerns, but did not fix the allocation size issue yet. Should not be hard to do, and it should be done.
I'm not sure how you plan to modify a zip file, whether you do it the "temp file" way or the "change in place" way, where a process crash would not create a corrupted version of the file.
Traditionally, you would do this by creating a temporary zip file, and doing a "mv" on it, so the name changed to the new name, but no data needed to be moved. I'm not sure you can do this in Java, unless you had planned on shelling out the OS (and detecting different OSes to do this).
There were two main reasons I wanted to do this in memory:
1) So you would not need any type of temp directory
2) The performance differences can be VAST in some cases. Imagine dealing with large XML or text files, or anything where the compression is significant. If you do the temp file approach, you need to uncompress everything, and write it to a file. Then, re-open all those files and re-compress them. This is not only a lot more CPU time in compressing and uncompressing, but also a lot more data-movement using the disk drive.